Osteomacs are macrophages residing in bone. They are known to actively participate in bone formation, healing and hematopoietic stem cell niche, but their role in skeletal metastasis is not fully elucidated. Preliminary data from the project laboratory support macrophages as tumor promoting cells. In the absence of macrophages, apoptotic cells are increased in subcutaneous tumors. Interestingly, tumors have a high rate of cells undergoing apoptosis that is usually not detected because macrophages actively participate as scavenger cells phagocytosing the apoptotic cells, an underexplored mechanism also known as efferocytosis. Macrophages secrete proteins, such as milk fat globule-EGF factor 8 (MFG-E8), that binds to both apoptotic and phagocytic cells, facilitating efferocytosis. In addition to apoptotic cell clearance MFG-E8 also modulates macrophage signaling pathways, with SOCS3 activation and inhibition of pro-inflammatory pathways, a potential mechanism for macrophage polarization into an M2 tumor promoting macrophage. The hypothesis for this F32 award is that macrophages support prostate cancer growth in bone mediated by phagocytosis/efferocytosis of apoptotic tumor cells. We propose to investigate the efferocytosis of tumor cells mediated by MFG-E8, its effects on macrophage immune modulation and the contribution to skeletal metastasis Utilizing potent animal models of macrophage and MFG-E8 ablation as well as therapeutic strategies to alter MFG-E8 levels we propose 2 different aims. The first aim will investigate the role of efferocytosis of apoptotic tumor cells via MFG-E8 and it impact in macrophage activation into tumor-promoting macrophages in vitro. The second aim will determine the contributions of macrophages and efferocytosis for tumor growth in bone in vivo. These studies will identify mechanisms of macrophage recruitment and their role in skeletal metastasis. This research is critical and relevant because it will advance the understanding of mechanisms mediating immune cell interactions with metastatic cells and hence improve development of new therapeutics targets.